Memory controller having receiver circuitry capable of alternately generating one or more data streams as data is received at a data pad, in response to counts of strobe edges received at a strobe pad

ABSTRACT

A double data rate memory controller is provided with a plurality of data and strobe pads, means for receiving data and strobe signals via said pads at 1x double data rate memory speed, and means for receiving data and strobe signals via said pads at Mx double data rate memory speed (M2).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/828,604filed on Apr. 7, 2001, now U.S. Pat. No. 6,633,965 issued Oct. 14, 2003,which patent is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention pertains to the field of memory controllers.

BACKGROUND OF THE INVENTION

The purpose of a memory controller is to field and execute memory accessrequests (i.e., requests to read data from, and write data to, a numberof memory modules). A memory access request may be initiated by either acentral processing unit (CPU) or an input/output device (I/O device).

In the past, most memory controllers have been designed to access memorymodules which are read and written via common clock data transmissions.That is, data bits are transmitted between a memory controller and anumber of memory modules in sync with the rising edges of the memorycontroller's internal clock. However, there is a current push to designmemory controllers which are capable of accessing double data rate (DDR)memory modules.

A DDR memory module is one which is read and written via sourcesynchronous data transmissions. That is, data bits are transmittedbetween a memory controller and a number of memory modules in sync withthe rising and falling edges of a strobe, with the strobe beinggenerated by the component which sources the data. The strobe is thenused by the component which receives the data for the purpose ofcapturing the data. Thus, a strobe is transmitted by the memorycontroller during a write operation, and a strobe is transmitted by amemory module during a read operation.

SUMMARY OF THE INVENTION

As is known by those skilled in the art, the complexity of memorycontrollers makes them very expensive components to design, develop andverify. The inventors therefore provide below a description of a memorycontroller having a greater number of functional modes. By providing amemory controller with a greater number of functional modes, anapplication specific integrated circuit (ASIC) manufacturer can satisfya greater number of computing applications with a single memorycontroller, and thus save time and expense by designing, building andtesting a fewer number of memory controllers. From a computermanufacturing perspective, the use of a common memory controller in avariety of computer systems enables machine dependent code, printedcircuit board design, et cetera to be leveraged from one computer systemto the next.

One aspect of the invention is embodied in a double data rate memorycontroller that is provided with a plurality of data and strobe pads,means for receiving data and strobe signals via said pads at 1× doubledata rate memory speed, and means for receiving data and strobe signalsvia said pads at M× double data rate memory speed (M2).

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred embodiments of the invention areillustrated in the drawings, in which:

FIG. 1 illustrates a computer system;

FIG. 2 illustrates a first exemplary embodiment of FIG. 1's memorycontroller, wherein the memory controller is directly attached to aplurality of memory modules for the purpose of data transmissions in a1× mode;

FIG. 3 illustrates a second exemplary embodiment of FIG. 1's memorycontroller, wherein the memory controller is attached to a plurality ofmemory modules via an intermediary chip for the purpose of datatransmissions in a 2x mode;

FIG. 4 illustrates memory controller driver circuitry which is capableof driving data in a 1x mode or 2x mode;

FIG. 5 illustrates a preferred embodiment of the output mergingcircuitry shown in FIG. 4;

FIG. 6 illustrates a variation of the FIG. 4 memory controller drivercircuitry, wherein circuitry is provided for initiating a write phasedelay;

FIG. 7 illustrates a preferred embodiment of a clock circuit whichproduces many of the clock signals appearing in FIGS. 8, 12–19, and21–26;

FIG. 8 illustrates a preferred embodiment of memory controller drivercircuitry, including data driver circuitry and corresponding strobedriver circuitry;

FIG. 9 illustrates a memory subsystem, including a third exemplaryembodiment of FIG. 1's memory controller, wherein the memory controllercomprises a plurality of corresponding strobe and data pads;

FIG. 10 illustrates an exemplary manner of matching strobe and data padsin order to support a memory controller's write to x4, x8 or x16 DIMMs;

FIG. 11 illustrates a memory map for tracking whether x4, x8 or x16DIMMs are attached to the data and strobe pads of a memory controller;

FIG. 12 illustrates a preferred embodiment of a circuit for providingthe act_stb[0:1] signals (see FIG. 8) to lower strobe pads of the FIG. 9memory controller;

FIG. 13 illustrates a preferred embodiment of a circuit for providingthe act_stb[0:1] signals (see FIG. 8) to upper strobe pads of the FIG. 9memory controller;

FIG. 14 illustrates write timings of the FIG. 8 driver circuitry whenconfigured in 1x mode with aligned write strobes, wpd=0, andlong_wpre=0;

FIG. 15 illustrates write timings of the FIG. 8 driver circuitry whenconfigured in 1x mode with aligned write strobes, wpd=0, andlong_wpre=1;

FIG. 16 illustrates write timings of the FIG. 8 driver circuitry whenconfigured in 2x mode with delayed write strobes, wpd=0, andlong_wpre=0;

FIG. 17 illustrates write timings of the FIG. 8 driver circuitry whenconfigured in 2x mode with delayed write strobes, wpd=0, andlong_wpre=1;

FIG. 18 illustrates a preferred embodiment of a memory controller's datareceiver circuitry;

FIG. 19 illustrates a first preferred embodiment of a memorycontroller's strobe receiver circuitry;

FIG. 20 illustrates a second preferred embodiment of a memorycontroller's strobe receiver circuitry;

FIG. 21 illustrates a controller-memory-controller read path loop;

FIG. 22 illustrates read timings of the receiver circuitry shown inFIGS. 18, 19 and 28 when configured in 1x mode with rpd=0;

FIG. 23 illustrates greater details of the “early” 1x mode read caseillustrated in FIG. 22;

FIG. 24 illustrates greater details of the “late” 1x mode read caseillustrated in FIG. 22;

FIG. 25 illustrates read timings of the receiver circuitry shown inFIGS. 18, 19 and 28 when configured in 2x mode with rpd=0;

FIG. 26 illustrates greater details of the “early” 2x mode read caseillustrated in FIG. 25;

FIG. 27 illustrates greater details of the “late” 2x mode read caseillustrated in FIG. 25;

FIG. 28 illustrates a preferred embodiment of a circuit for providingthe set_alt_n signal which appears in the FIG. 18 memory controllerreceiver circuitry; and

FIG. 29 illustrates a relation between the core and pad circuitry of theFIG. 1 memory controller.

DESCRIPTION OF THE PREFERRED EMBODIMENT 1. In General: A MemoryController with a Greater Number of Functional Modes

A memory controller 100 with a greater number of functional modes isillustrated in FIGS. 1–3. The memory controller 100 is preferably adouble data rate (DDR) memory controller, but need not be. A DDR memorycontroller 100 is one which is capable of communicating with DDRsynchronous dynamic random access memories (SDRAMs). See, e.g., JEDECStandard No. 79 published June 2000, which is hereinafter referred to asthe “JEDEC DDR SDRAM Specification” (and which is hereby incorporated byreference).

FIG. 1 illustrates an exemplary computer system 124 in which the memorycontroller 100 may be used. The computer system 124 comprises a numberof central processing units 102 (CPUs) which are connected to the memorycontroller 100 over a system bus 106. As shown in FIG. 1, the memorycontroller 100 preferably forms part of an integrated memory andinput/output (I/O) controller (MIOC) 100. The MIOC 100 receives accessrequests over the system bus 106, and then addresses memory modules 104and/or I/O devices 112–122 in order to process the memory accessrequests. Fetched data is then returned as necessary. Inbound memoryaccess requests received from the I/O devices 112–122 may also beprocessed by the MIOC 100. As is known in the art, memory and I/O accessrequests typically comprise read requests and write requests. The MIOC100 is coupled to a number of memory modules 104 over a memory bus 108,and is coupled to I/O devices 112–122 via local buses, interfaces, etc.110 (e.g., a peripheral component interconnect (PCI) local bus, or anintegrated device electronics (IDE) interface). The memory modules maycomprise, for example, a number of DDR Dual In-Line Memory Modules(DIMMs). A DIMM is a fixed data width (usually 64 or 72 bits) collectionof RAM devices (e.g., DDR SDRAMs). I/O devices may comprise one or moreof the following, as well as other devices: drives 112 (e.g., harddrives, CD-ROM drives, floppy drives), ports 114 (e.g., USB, parallel,serial), a keyboard 116, a mouse 118 and/or other pointing devices, adisplay 120, and a printer 122.

It is important to note once again that FIG. 1 provides only oneexemplary embodiment of a computer system 124 in which the memorycontroller 100 described below may be used, and thus the FIG. 1 computersystem 124 is not meant to limit the invention and/or its applicableuses. It is also important to note that much of the followingdescription refers only to a “memory controller” 100. However, one ofordinary skill in the art will readily comprehend that the features of amemory controller which are disclosed below may be readily adapted foruse in a memory controller 100 forming part of the integrated “memoryand I/O controller” 100 illustrated in FIGS. 1–3.

In FIG. 2, the MIOC 100 of FIG. 1 is shown to be coupled directly to anumber of memory modules 104 via a 1x bus 200 (e.g., a 1x DDR bus). Asdefined herein, a 1x DDR bus 200 is a memory bus which operates in aconventional DDR mode, wherein data is transmitted in sync with bothedges of a strobe signal.

A. Alternate or Simultaneous x4, x8, x16 Attach

One feature of the memory controller 100 disclosed herein is that it canread and write data to non-homogeneous memory modules 104. To understandwhat is meant by “non-homogeneous memory modules”, a little backgroundis needed.

Memory modules 104 are available in a variety of configurations, themost popular of which is the Dual In-Line Memory Module (DIMM)configuration. Other configurations, of which there are many, includethe Single In-Line Memory Module (SIMM) configuration, and the SmallOutline DIMM (SO-DIMM) configuration.

A common characteristic of the afore-mentioned memory moduleconfigurations is that each comprises a printed circuit board with aplurality of random access memory (RAM) devices mounted thereon.Similarly to the various configurations of memory modules, RAM devicesmay also assume a variety of configurations, the most popular of whichis the SDRAM configuration. However, there is currently an industry pushto transition to memory modules populated with DDR SDRAM devices. By wayof example, FIG. 9 illustrates a plurality of DIMMs 104 a, 104 b, 104 cwhich are populated with DDR SDRAM devices 910, 912, 914.

DDR SDRAM devices are currently available in three data widths, withdevices of additional data widths being proposed. The currentlyavailable data widths are 4, 8 and 16 bits. As discussed in the JEDECDDR SDRAM Specification, a 4-bit wide DDR SDRAM is known as a x4 DDRSDRAM and is characterized by its generation/receipt of four datasignals in response to a single strobe signal. Likewise, an 8-bit wideDDR SDRAM is known as a x8 DDR SDRAM and is characterized by itsgeneration/receipt of eight data signals in response to a single strobesignal; and a sixteen bit wide DDR SDRAM is known as a x16 DDR SDRAM andis characterized by its generation/receipt of sixteen data signals inresponse to a pair of strobe signals. As is known by those skilled inthe art, the interface of a x16 DDR SDRAM is similar to that of a x8 DDRSDRAM in that eight data signals are generated/received in response toeach one of a x16 DDR SDRAM's strobe signals.

In a typical computer system, a memory controller 100 is capable ofaccessing a number of like memory modules which are inserted into aplurality of sockets 902–908 on the computer system's motherboard.Often, a computer manufacture will pre-load a number of the sockets902–908 with a number of like memory modules, and then instruct thecomputer system's end user that pre-loaded memory modules may be addedto, removed or swapped so long as all of the computer's memory modulesare of a specified, homogeneous module and RAM configuration (e.g.,DIMMs comprised of SDRAMs). If a memory module is not of the specifiedmodule and RAM configuration, the computer system's memory controllerwill be unable to communicate with the memory module, and in someinstances, the non-homogeneous memory module and/or the memorycontroller itself may be damaged. A computer user's memory upgradeoptions are therefore limited to one particular configuration of memorymodule. It would be desirable, however, if a computer user had moreflexibility when upgrading his or her computer memory.

As a result, there is disclosed herein a memory controller 100 which iscapable of reading and writing non-homogeneous memory modules 104. Thememory modules 104 are non-homogeneous in that they comprise RAM devicesof differing data widths. For example, two memory modules may comprisex8 DDR SDRAMs, a third memory module may comprise x4 DDR SDRAMs, and afourth memory module may comprise x16 DDR SDRAMs. The disclosed memorycontroller 100 communicates with the non-homogeneous memory modules 104by storing and accessing a memory map 1100 (FIG. 11) of RAM device datawidths, wherein a RAM device data width is stored for each of a computersystem's memory modules 104. An access to the memory map 1100 is made“on the fly” prior to each read and write cycle. Thus, when a write to ax4 DDR SDRAM located on a first memory module 104 a is followed by awrite to a x8 DDR SDRAM located on a second memory module 104 b, thememory controller 100 can perform the writes successively by 1)accessing the memory map 1100 prior to each write operation, and then 2)reconfiguring data and strobe driver circuitry as necessary.

To enable the memory controller's communication with a plurality ofnon-homogeneous memory modules 104, all that needs to be considered atthe board level is that enough data and strobe routes are provided forthe purpose of enabling an expansion socket's electrical connection tomemory modules 104 comprised-of varying data width RAM devices. Thus,when designing with the disclosed memory controller 100 in a DDR SDRAMenvironment, a computer manufacturer can 1) route fewer strobe signalsto/from a memory controller 100 and provide a computer user with theability to simultaneously use DIMMs comprised of x8 and x16 DDR SDRAMsin their computer system 124, or 2) route a few additional strobesto/from a memory controller 100 and provide a computer user with theability to simultaneously use DIMMs comprised of x4, x8 and x16 DDRSDRAMs 104 in their computer system 124. In either case, a computer useris provided with more flexibility to mix and match DIMMs than iscurrently provided.

A memory controller 100 that can read and write non-homogeneous memorymodules 104 (i.e., memory modules comprised of non-homogeneous datawidth RAM devices) is advantageous in that it provides a computer userwith a variety of memory upgrade options. For example, x4 DDR SDRAMs arehalf as wide but twice as deep as x8 and x16 DDR SDRAMs. Thus, one candouble their computer's memory capacity by using DIMMs populated with x4DDR SDRAMs in lieu of DIMMs populated with x8 or x16 DDR SDRAMs.However, given that DIMMs comprised of x8 SDRAMs are currently lessexpensive, one might wish to sacrifice some level of performance infavor of lower cost. Furthermore, a user might wish to add highercapacity DIMMs comprised of x4 DDR SDRAMs to his or her computer system,but still keep and use the existing x8 or x16 DDR SDRAMs which camepre-loaded with his or her computer system.

Absent the memory controller 100 disclosed herein, the memory capacityof a computer system which only accepts DIMMs comprised of x8 and/or x16DDR SDRAMs can only be increased through an increase in the number ofloads per memory data bit (e.g., double or quadruple the number ofloads). By so doing, the same memory capacity that can be achieved in ax4 system can be achieved in a x8/x16 system. A problem, however, isthat with more loads per bit, the maximum operating frequency of eachDIMM is decreased. Greater memory capacity is therefore achieved with aperformance penalty. Memory operations initiated by the memorycontroller 100 disclosed herein are not subject to such performancepenalties.

B. 1x or 2x Mode

Another feature of the memory controller 100 which is disclosed hereinis that it can generate strobes and data (i.e., write) in either a 1xmode or Mx mode (where M≧2 and x is a baseline rate at which data isread and written). Likewise, it can receive data and strobes (i.e.,read) in either a 1x mode or Mx mode.

In 1x mode, the memory controller 100 attaches directly to a pluralityof memory modules 104 as illustrated in FIG. 2. In Mx mode, however, thememory controller 100 attaches to one or more intermediate chips 302 viaa bus 300 which operates at an Mx speed. In Mx mode, reads and writesbetween the memory controller 100 and intermediate chips 302 occur at anMx rate. However, reads and writes between the intermediate chips 302and memory modules 104 continue to occur at a 1x rate. The advantage ofusing the intermediate chips 302 is that one can again double a system'smemory capacity—this time by 1) using the intermediate chips 302 toreceive 2x data and then distribute the 2x data in a 1x fashion to twobanks of memory modules 104, or 2) using the intermediate chips 302 toreceive 1x data from two banks of memory modules 104 and then multiplexthe data to provide it to a memory controller 100 at a 2x rate.

Use of the intermediate chips 302 also allows one to double a memorysystem's bandwidth.

C. Read and Write Phase Delays

The memory controller 100 which is disclosed herein further provides anability to delay read and write cycles by a single phase of the memorycontroller's internal clock. Thus, in addition to allowing read andwrite cycles to be tuned with full-cycle resolution, read and writecycles may be tuned with half-cycle resolution. Read and write cyclesmay therefore begin on either a rising or falling clock edge of thememory controller's internal clock. This feature provides a degree oftunability for the memory controller 100.

D. Tri-state Noise Immunity

During a memory read cycle, there is a need to account for variation incontroller-memory-controller loop delay (i.e., read loop delay). Forexample, in FIG. 21 a plurality of memory modules 104 is coupled to amemory controller 100 over common data (DQ) and strobe (DQS) buses. Notonly is a plurality of memory modules 104 coupled to the data and strobebuses, but each of the memory modules 104 may exhibit timing variationswithin allowed ranges (e.g., within the ranges provided in the JEDEC DDRSDRAM Specification). Furthermore, copies of a clock signal which aredistributed to each of the plurality of memory modules 104 may becomeskewed with respect to one another.

As a result of the above irregularities, read requests which aredispatched to different memory modules (with their varied timingcharacteristics and skewed clocks) can take varying amounts of time toreturn to the controller 100. As a result, there is a variation in readloop delay which needs to be accounted for when determining when toenable and disable the receipt of data and strobe signals at a memorycontroller 100. Such a delay can only be accounted for by ensuring thata memory controller 100 will appropriately receive data and strobes inresponse to a shortest possible loop delay (i.e., an early receipt case)and a longest possible loop delay (i.e., a late receipt case).

The data and strobe bus for memory modules 104 under the JEDEC DDR SDRAMSpecification have a notable characteristic. The reference voltage foreach bus line is the same as the bus line's termination voltage. Whatthis means is that, as a result of noise, the strobe pads of a memorycontroller 100 are subject to erroneous “0” to “1” and “1” to “0”transitions when their corresponding bus lines are tri-stated. If notaccounted for, these transitions can be erroneously interpreted asactive strobe edges, thereby leading to potential data corruption.

The memory controller 100 disclosed herein solves the above problem bycounting incoming strobe edges which are received at a strobe pad, andthen using a count of the strobe edges to control a plurality of datalatches which are coupled to a data pad. When a count representing anexpected number of incoming strobe edges is reached, no further countingis undertaken, and noise which is produced as a result of a strobe bustri-stating is prevented from latching additional data into theplurality of data latches.

2. Ability to Write in 1x or 2x Mode

FIG. 4 illustrates a first preferred embodiment of driver circuitry 400for a memory controller 100. The driver circuitry 400 comprises a datapad (DQ4), two data propagation circuits 402, 404, a multiplexing stage406, and output merging circuitry 408. A first data stream 410 isprovided to each of the data propagation circuits 402, 404, and a seconddata stream 412 is optionally provided to the second of the two datapropagation circuits 404. The second data propagation circuit 404receives either the first or second data stream 410, 412 via themultiplexing stage 406, which in FIG. 4 consists of a singlemultiplexer. The multiplexer 406 is controlled by a 2x mode signal(data2xn1×) which enables the first data stream 410 to be coupled to themultiplexer's output in a 1x mode of operation, and enables the seconddata stream 412 to be coupled to the multiplexer's output in a 2x modeof operation. In either mode, data propagates through each of the datapropagation circuits 402, 404 to the output merging circuitry 408, atwhich point the two data propagation circuits 402, 404 are alternatelycoupled to the data pad to thereby generate either a 1x or 2x stream ofdata bits. Details of the output merging circuitry 408 will be discussedlater in this description.

Functionally, the driver circuitry 400 depicted in FIG. 4 operates asfollows. In 1x mode, the same data stream 410 is provided to each of thedata propagation circuits 402, 404. As a result, a data bit whichpropagates through the first data propagation circuit 402 will appear atthe data pad when the output merging circuitry 408 couples the firstdata propagation circuit 402 to the data pad. The same data bit willthen appear at the data pad again when the output merging circuitry 408later couples the second data propagation circuit 404 to the data pad.As a result, data bits will appear at the data pad at the same ratewhich they are provided to the driver circuitry 400 (i.e., in a 1xmode).

In the FIG. 4 driver circuitry's 2x mode of operation, a first datastream 410 is provided to the first data propagation circuit 402 and asecond data stream 412 is provided to the second data propagationcircuit 404. The first data stream 410 may comprise, for example, bits0, 2, 4, . . . of a data stream, while the second data stream 412 maycomprise, for example, bits 1, 3, 5, . . . of a data stream. As aresult, different data bits propagate through each of the first andsecond data propagation circuits 402, 404, and the output mergingcircuitry 408 produces a 2x data stream of bits 0, 1, 2, . . . at thedata pad. Note that the output data stream is considered a 2x datastream because it produces data bits at twice the rate of either of thedriver circuitry's data input streams 410, 412.

An exemplary embodiment of the output merging circuitry 408 is disclosedin FIG. 5. The circuitry 408 comprises two D-type flip-flops 500, 502,each of which receives data from one of FIG. 4's two data propagationcircuits 402, 404. The flip-flops 500, 502 are alternately clocked onthe positive and negative edges of a clock IOBCK. The output of eachflip-flop 500, 502 is respectively received by a tri-statable buffer504, 506. Each buffer 504, 506 also receives the clock IOBCK, and isoperated in sync with its corresponding flip-flop 500, 502. Thus, whendata is clocked out of flip-flop 500, buffer 504 allows the data to passthrough to data pad DQ4, and buffer 506 is tri-stated. Likewise, whendata is clocked out of flip-flop 502, buffer 506 allows the data to passthrough to data pad DQ4, and buffer 504 is tri-stated. The twoflip-flops 500, 502 and buffers 504, 506 therefore alternately providedata to the data pad DQ4. Each corresponding flip-flop and buffer inFIG. 5 are an example of a tri-statable path, as referenced in theclaims.

Note that the output merging circuitry 408 could also comprise amultiplexer in lieu of the flip-flops 500, 502 and buffers 504, 506.However, the arrangement set forth in FIG. 5 is preferred, because theC→Q time is minimized. In fact, it is preferable to minimize the C→Qtime even further by integrating the functionality of the buffers 504,506 into the flip-flops 500, 502.

FIG. 6 illustrates a second preferred embodiment of driver circuitry 600for a memory controller 100. Like the driver circuitry 400 illustratedin FIG. 4, the FIG. 6 driver circuitry 600 comprises a data pad (DQ4),two data propagation circuits 602, 604, a multiplexing stage 606, andoutput merging circuitry 608. However, the driver circuitry 600additionally comprises a phase delay circuit 614 and first and secondphase delay multiplexers 616, 618. The purpose of the additional phasedelay circuitry 614–618 is to delay the appearance of data at the datapad and thereby incur a “write phase delay”, as might be required totune a system for maximum margin. The ability to delay a write using thephase delay circuitry 614–618 therefore provides the driver circuitry600 with a tunability feature.

The first phase delay multiplexer 616 receives both first and seconddata streams 610, 612, and in response to the data2xn1x signal, selectseither the first 610 or second 612 data stream for output to the phasedelay circuit 614. The second phase delay multiplexer 618 receives dataoutput from both the first data propagation circuit 602 and the phasedelay circuit 614 and determines which data to output to the outputmerging circuitry 608 in response to the exclusive-OR 620 (XOR) of thedata2xn1x signal with a write phase delay (wpd) signal. Note that thedata2xn1x signal is asserted in 2x mode, and not asserted in 1x mode.However, the orientation of the wpd signal switches depending on thestate of the data2xn1x signal. In 1x mode, the wpd signal is assertedfor a write phase delay, and not asserted for no write phase delay. In2x mode, the wpd signal is asserted for no write phase delay, and notasserted for the purpose of incurring a write phase delay.

Note that in FIG. 6, the multiplexing stage multiplexer 606 is no longercontrolled by the data2xn1x signal, but is instead controlled by the AND622 of the data2xn1x signal and the wpd signal.

Functionally, the FIG. 6 driver circuitry 600 operates as follows. Inboth 1x and 2x modes with no write phase delay (wpd=0 in 1x mode; wpd=1in 2x mode), data propagates through the first and second datapropagation circuits 602, 604, with data propagating through the firstdata propagation circuit 602 and output merging circuitry 608 first. Inboth 1x and 2x modes with a write phase delay (wpd=1 in 1x mode; wpd=0in 2x mode), data propagates through the second data propagation circuit604 and the phase delay circuit 614, with data propagating through thesecond data propagation circuit 604 and output merging circuitry 608first. Note that with a write phase delay in either 1x or 2x mode, datamay still propagate into the phase delay circuit 614. However, unless awrite delay is indicated, the data which propagates into the phase delaycircuit 614 will not propagate through the second phase delaymultiplexer 618 and onto the data pad DQ4.

One of ordinary skill in the art will readily understand how each of thedriver circuits 400, 600 disclosed in FIGS. 4 and 6 can be extrapolatedto provide driver circuitry which is capable of operating in either a 1xor Nx mode. With respect to extrapolating the FIG. 6 circuitry for N>2,one will note that only a single phase delay circuit 614 is needed,regardless of the number of data propagation circuits 602, 604 which areadded to the driver circuitry 600.

One of ordinary skill in the art will also understand how extrapolateddriver circuitry (where N is an even number ≧2) can be used to supply anMx data stream where 1<M≦N.

FIGS. 7 and 8 illustrate an enhanced embodiment 800 of the FIG. 6 drivercircuitry, wherein the data pad DQ4 may be tri-stated, and wherein astrobe which is produced at a strobe pad DQS18 is synchronized with theoutput of data at the data pad.

FIG. 7 illustrates a preferred embodiment of a clock circuit 700 whichproduces many of the clock signals appearing not only in FIG. 8, butalso in FIGS. 12–20, and 22–27. The clock circuit 700 is driven by acore clock, MCK, of a memory controller. The clock circuit in turnoutputs clocks IOBCK, MUX_CLK and IOSCK. Clock IOBCK is merely abuffered version of the MCK clock (buffered through a buffer 702). ClockMUX_CLK is buffered through a buffer 704, but is produced at either therate of the MCK clock (i.e., when multiplexer 706 is configured for 1xmode operation) or at ½ the rate of the MCK clock (i.e., as a result ofthe divider 708 through which the MCK clock passes when multiplexer 706is configured for 2x mode operation). Clock MUX_CLK may be provided toeither a multiplexer chip 302 or a memory module 104 (see FIGS. 2 and3). If multiplexer 710 is configured for 1x mode operation, clock IOSCKis equivalent to clock MCK. However, when multiplexer 710 is configuredfor 2x mode operation, clock IOSCK represents a version of MCK which isdelayed by ¼ period (i.e., as a result of ¼ Period Delay circuitry 712).Clock IOSCK enables the FIG. 8 driver circuitry to provide appropriate2x mode strobe signals to a preferred embodiment of an intermediary chip302 (FIG. 3).

The driver circuitry 800 illustrated in FIG. 8 shares many similaritieswith the driver circuitry 600 illustrated in FIG. 6. The correspondenceof components between FIGS. 6 and 8 is as follows. The first datapropagation circuit 602 in FIG. 6 corresponds to a simple wire route 802in FIG. 8; the second data propagation circuit 604 corresponds to afirst D-type flip-flop 804; and the phase delay circuit 614 correspondsto a second D-type flip-flop 806. The correspondence of other FIG. 6components is indicated in FIG. 8 by the use of like reference numbers.

The output merging circuitry illustrated in FIG. 8 is similar to thatwhich is disclosed in FIG. 5 and comprises two D-type flip-flops 808,810 and two tri-statable buffers 809, 811.

If data may be alternately written and read through a data pad (e.g.,data pad DQ4), then the ability to tri-state the pad's write path duringreads may be desirable. Thus, FIG. 8 incorporates a tri-state buffer 812between the output merging circuitry 808, 810 and the data pad. Thestate of the buffer 812 is changed during the commencement andcompletion of writes using two signals: trist_d and wpd. The purpose ofthe wpd signal has already been discussed, supra. The trist_d signal isprovided to a multiplexer 814 after incurring a delay through one or twoD-type flip-flops 816, 818. These flip-flops 816, 818 are constructedand clocked similarly to flip-flops 804, 810 found in the output mergingcircuitry and second data propagation path. The state of the multiplexer814 is controlled by the wpd signal such that the trist_d signal opensthe tri-state buffer 812 in sync with the output merging circuitry'sinitial output of data from either the first data propagation path 802or the second data propagation path 804 (i.e., when wpd is asserted(wpd=1 in 1x mode; wpd=0 in 2x mode), the output of data at pad DQ4 isdelayed by ½ the period of clock IOBCK).

The JEDEC DDR SDRAM Specification dictates that data is to betransmitted synchronously with a strobe. As a result, the FIG. 8 drivercircuitry is provided with a strobe pad (DQS18), and logic 824–834 forgenerating a strobe signal which is appropriately matched to the 1x or2x data provided at the DQS18 data pad.

Circuitry 836, 838, 840 which is similar to that which enables thetri-state buffer 812 coupled to the data pad DQS may, be used to operatethe tri-state buffer 822 coupled to the strobe pad DQS18. However, fortiming considerations which will be described later in this description,the circuitry is controlled by the wpd signal and a trist_s signal whichfunctions separately, but similarly, to the trist_d signal.

A strobe is generated by providing a pair of signals, act_stb[0] andact_stb[1], to the strobe driver circuitry 824–834. In 2x mode with nowrite phase delay, act_stb[0] is forced low and act_stb[1] is forcedhigh for the duration of a write cycle. The act_stb[0:1] signals arethen respectively clocked through first and second pairs of D-typeflip-flops 824/826, 832/834. The act_stb[0:1] signals are clockedthrough the first pair of flip-flops 824, 826 in parallel, but theact_stb[1] signal is clocked through the second pair of flip-flops 832,834 first. Thus, by inverting the act_stb[0:1] signals during a 2x modewrite with write phase delay, the first clocking of flip-flop 834 willhold the DQS18 output low for an additional ½ clock cycle (i.e., onephase) and delay the appearance of a strobe at the DQS18 output for ½clock.

Note that as in the data driver circuitry, each flip-flop 832, 834 isfollowed by a tri-statable buffer 833, 835.

The states of act_stb[0] and act_stb[1] are therefore static during a 2xwrite. However, this is not the case in 1x mode.

In 1x mode, each of the act_stb[0:1] signals toggle at a 1x rate, and awrite phase delay is implemented by merely delaying the first rise ofeach of the act_stb[0:1] signals.

Circuits which may be used for both 1) generating the act_stb[0] andact_stb[1] signals, and 2) implementing additional strobe functionalitywhich has yet to be described, will be discussed in the next section ofthis description.

3. Ability to Write DIMMs Comprised of x4, x8 and x16 RAM Devices

The JEDEC DDR SDRAM Specification specifies that DDR SDRAMs may beconstructed as x4, x8 or x16 devices. Writes to x4 DDR SDRAMs requireone strobe signal for each set of four data signals (i.e., a 4:1data/strobe ratio), while writes to x8 and x16 DDR SDRAMs require onestrobe signal for each set of eight data signals (i.e., an 8:1data/strobe ratio).

In the past, DDR memory controllers have been designed to communicatewith one type of DDR memory module (i.e., a set of homogeneous memorymodules comprised only of x4, x8 or x16 DDR SDRAMs). However, a DDRmemory controller would offer greater flexibility, particular when acomputer user desires to upgrade his or her computer memory, if thememory controller were capable of communicating with memory modulescomprised of non-homogeneous data width RAM devices. To fill this need,the memory controller illustrated in FIGS. 7–13 is capable of writing tomemory modules comprised of non-homogeneous data width RAM devices(e.g., DIMMs comprised of x4 DDR SDRAMs, DIMMs comprised of x8 DDRSDRAMs, and DIMMs comprised of x16 DDR SDRAMS).

FIG. 9 illustrates an exemplary environment (i.e., a memory interface900) in which the memory controller 100 may operate. Note that thememory controller 100 is coupled to a plurality of sockets 902–908 viacommon data and strobe lines. A first of the sockets 902 holds a DIMM104 a comprised of x4 DDR SDRAMs; a second of the sockets 904 holds aDIMM 104 b comprised of x8 DDR SDRAMs; and a third of the sockets 906holds a DIMM 104 b comprised of x16 DDR SDRAMS 910–914.

In FIG. 9, the memory controller 100 is illustrated to have a pluralityof strobe pads, each of which is associated with a plurality of datapads. By way of example, and to offer seamless operation with existingDDR DIMMs, each strobe pad is shown to be associated with 4 data pads(e.g., strobe pad DQS0 corresponds to data pads DQ0–DQ3, and strobe padDQS18 corresponds to data pads DQ4–DQ7). Although only two strobe andeight data pads are illustrated in FIG. 9, the memory controller 100might comprise, for example, 36 strobe pads and 144 data pads.

As will be described in greater detail below, when communicating withDIMMs comprised of x4 DDR SDRAMs, the memory controller 100generates/receives signals at each of its data and strobe pads. As aresult, there is a 4:1 correspondence between data and strobe signalswhen the memory controller 100 communicates with DIMMs comprised of x4DDR SDRAMs. However, when communicating with DIMMs comprised of x8 orx16 DDR SDRAMs, the memory controller 100 generates/receives data ateach of its data pads, but only generates/receives strobes at its lowerstrobe pads (i.e., strobe pads DQS0–DQS17). As a result, there is an 8:1correspondence between data and strobe signals when the memorycontroller communicates with DIMMs comprised of x8 or x16 DDR SDRAMs.FIG. 10 illustrates an exemplary mapping of strobe pads to data pads forthe FIG. 9 memory controller, depending on whether the memory controller100 is driving data to DIMMs comprised of x4 or x8/x16 DDR SDRAMs. Notethat the memory controller's upper strobe pads are held low duringwrites to DIMMs comprised of x8 or x16 DDR SDRAMs.

A determination as to whether data is being written to a DIMM comprisedof x4 or x8/x16 DDR SDRAMs may be made by maintaining a memory map 1100(FIG. 11) within the FIG. 9 memory controller. Such a map 1100 maycomprise a table of corresponding DIMM locations 1104 (e.g., sockets)and DIMM types, wherein the types specify, for example, 1) indicationsof RAM device data widths 1106 for a number of DIMMs, or 2) indicationsof data/strobe ratios for a number of DIMMs. The indications maycomprise, for example, a value A_(x) for each memory module x which iscoupled to the memory controller 100.

The values A_(x) stored in the memory map may be variously embodied.However, for the purpose of communicating with DIMMs 104 comprised ofx4, x8 and x16 DDR SDRAMs, each value A_(x) may consist of a singlebinary bit, the two values of which represent the data/strobe ratioswhich are required to read and write x4 and x8/x16 DDR SDRAMs,respectively. For example, a logic “1” might represent a 4:1 data/stroberatio, as required of DIMMs comprised of x4 DDR SDRAMs, and a logic “0”might represent an 8:1 data/strobe ratio.

Alternatively, each value A_(x) could be a binary equivalent of anactual data/strobe ratio. For example, a 4:1 data/strobe ratio could bestored in the memory map as the value “0100”, while an 8:1 data/stroberatio could be stored in the memory map 1100 as the value “1000”. Giventhe data/strobe ratios assumed by x4, x8 and x16 DDR SDRAMs, the storageof four bit values in a memory map 1100 is unnecessary. However,application of the above principles to non-DDR environments, and/or tofuture DDR environments, might make the storage of binary equivalents ofdata/strobe ratios more desirable.

Each value A_(x) could also be (or represent) the data width of RAMdevices 910–914 mounted on a DIMM 104 c. However, if RAM devices ofdiffering data widths have the same data/strobe ratios, as in the caseof x8 and x16 DDR SDRAMs, the size of A_(x) values may be reduced ifeach value A_(x) merely represents the data/strobe ratio of RAM devicesmounted on a DIMM.

The memory map 1100 may be maintained by initializing it upon boot orreconfiguration of a computer system 124. In a preferred embodiment, aRAM device data width is read from each memory module coupled to thememory controller 100, and each RAM device data width is then used togenerate a value which is stored in the memory map 1100. Alternatively,although not preferred, the data widths retrieved from the memorymodules 104 (or representations thereof) may be stored directly in thememory map 1100. If the memory modules 104 coupled to the memorycontroller 100 are DDR memory modules, then each memory module maymaintain a DDR SDRAM data width in a serial presence detect ROM 916located on the memory module. If the memory controller 100 executes aserial presence detect sequence within the memory modules 104, then aDDR SDRAM data width stored in a ROM of each memory module may be read,converted to an appropriate value A_(x), and stored in the memory map1100.

The memory map 1100 may also be maintained by providing it with a valueA_(x) for each memory module via a user interface (e.g., the bios SETUPutility of a computer system).

During a write cycle, the memory map 1100 is addressed by all or part ofa memory address, and an addressed value A_(x) is output from the memorymap 1100. The output value is then used to determine, “on the fly”, 1)how many strobes need to be generated by the memory controller 100, and2) where the strobes need to be generated (i.e., at which strobe pads).

During write cycles of the memory controller 100, addressed values A_(x)are received by strobe driver circuitry comprising two or more subsetsof strobe driver circuits, wherein each strobe driver circuit may beconfigured as illustrated in FIG. 8. The subsets of strobe drivercircuits are configured such that at least one of the subsets generatesstrobes in response to only a portion of said values A_(x). For example,if each value A_(x) consists of a single binary bit, one subset ofstrobe driver circuits might only generate strobes when A_(x)=1, whilethe other subset of strobe driver circuits might generate strobes forboth values of A_(x). Referring to FIG. 10, strobes are always generatedat lower strobe pads, but strobes are only generated at upper strobespads when the memory map outputs a logic “1” (i.e., when A_(x) is alogic “1”). In this manner, strobes are generated at all strobe padswhen an addressed value A_(x) is indicative of a 4:1 strobe ratio, andstrobes are generated at only half of the strobe pads (i.e., a subset ofstrobe pads consisting of the lower strobe pads) when A_(x) isindicative of a 8:1 strobe ratio.

The memory map 1100 preferably forms part of a larger memory addressrouter (MAR) 1102. The MAR 1102 may comprise other information regardingthe type and organization of memory modules 104 coupled to the memorycontroller 100, in addition to supporting circuitry. When the MAR 1102is provided with a memory address, the memory map 1100 and other tablesare accessed to determine the DIMM socket and DIMM bank in which theaddress is located. A row and column address within the addressed DIMMis also determined. At the same time, a data/strobe ratio is accessed sothat the strobe pads of the memory controller 100 illustrated in FIG. 9may be appropriately configured for writing or receiving data from theDIMM type which is being addressed.

The driver circuitry for the various data and strobe pads shown in FIG.9 may be implemented as shown in FIG. 8. In such an implementation, itis the act_stb[0] and act_stb[1] signals which determine if, when andhow a signal is generated at a strobe pad. Circuitry is therefore neededfor generating two sets of the act_stb[0] and act_stb[1] signals.Circuitry 1200 for generating the set of act_stb[0] and act_stb[1]signals which are needed to configure the lower strobe pads (i.e., padsDQS0–DQS17) of the FIG. 9 memory controller is illustrated in FIG. 12,and circuitry 1300 for generating the act_stb[0] and act_stb[1] signalswhich are needed to configure the upper strobe pads (i.e., padsDQS18–DQS35) of the FIG. 9 memory controller is illustrated in FIG. 13.In FIG. 12, the act_stb[0] and act_stb[1] signals of FIG. 8 have beenrespectively renamed act_stb_low[0] and act_stb_low[1]. Likewise, inFIG. 13 the act_stb[0] and act_stb[1] signals of FIG. 8 have beenrespectively renamed act_stb_up[0] and act_stb_up[1].

Note that the circuitry 1200, 1300 illustrated in FIGS. 12 and 13 iscapable of operating in several modes, including 1x or 2x mode, andmodes with or without a write phase delay. In addition, x4 and x8 writemodes, either with or without a long write preamble, can be achieved. Ifa memory controller 100 with less functionality is desired, one ofordinary skill in the art will readily understand how to eliminate gatesin the FIG. 12 and 13 circuits to thereby eliminate functionality whichis not needed for a given application.

Operation of the FIG. 12 circuitry in 1x mode will now be described. In1x mode, the data2xn1x signal is driven low, and the assertion of thewrite_m_active signal begins the generation of act_stb_low[0:1] signals.When the data2xn1x signal is driven low, the multiplexers 1202, 1204,1226 which are controlled thereby output the data which is received attheir “0” inputs. When the write_m_active signal is asserted,act_stb_low[0] begins to toggle at the frequency of clock MCK, yieldinga signal of frequency MCK/2, due to the arrangement of gate 1206,multiplexer 1202, D-type flip-flop 1208, and feedback path 1210.Likewise, the assertion of the write_m_active signal causesact_stb_low[1] to toggle. For timing considerations, a gate 1212 isinserted in the act_stb_low[1] path. The gate 1212 receives the feedbacksignal 1210 and the write_m_active signal, and when the write_m_activesignal is high, outputs the feedback signal 1210, and a version thereofwhich is delayed through a flip-flop 1214, to the inputs of anadditional multiplexer 1216. The multiplexer 1216 is controlled by thewpd signal in order to propagate the feedback signal 1210 through to theact_stb_low[1] output with or without a delay. In the case of no writephase delay, act_stb_low[1] propagates through to the strobe pad DQS18first (see FIG. 8). Otherwise, act_stb_low[0] propagates through to thestrobe pad first. Thus, the assertion of the wpd signal causes a ½ cyclewrite phase delay to be incurred.

The FIG. 12 circuitry operates in 2x mode as follows. The data2xn1xsignal is driven high, and the multiplexers 1202, 1204, 1226 which arecontrolled thereby output the data which is received at their “1”inputs. By means of flip-flop 1218 and gate 1220, act_stb_low[0] isasserted when write_m_active_(—)2x is high and wpd is low (i.e., whenthere is a 2x write with a write phase delay). By means of gate 1222 andflip-flop 1224, act_stb_low[1] is asserted when write_m_active_(—)2x andwpd are both high (i.e., when there is a 2x write with no write phasedelay).

Note that the FIG. 12 circuitry is not able to achieve a long writepreamble. However, a long write preamble may be achieved at a lowerstrobe pad, either in 1x or 2x mode, by asserting the trist_s signal onecycle early (see FIG. 8).

In summary, the FIG. 12 circuitry produces outputs which toggle in 1xmode, and produces outputs which are static opposites in 2x mode. Thetoggling outputs are used by the FIG. 8 circuitry to produce a 1xstrobe, and the static outputs are used by the FIG. 8 circuitry toproduce a 2x strobe.

The operation of FIG. 13 in 1x mode will now be described. In 1x mode,the data2xn1x signal is driven low, and the assertion of thewrite_m_active signal enables the generation of act_stb_up[0:1] signals.However, act_stb_up[0:1] signals are only generated when a write isbeing made to a DIMM comprised of x4 DDR SDRAMs. When writing to DIMMscomprised of x8 or x16 DDR SDRAMs, the act_stb_up[0:1] signals aredriven low so that no strobes are produced at the upper strobe pads ofthe FIG. 9 memory controller. Thus, FIG. 13 needs to be analyzed withrespect to a x4 write in 1x mode, and a x8/x16 write in 1x mode.

During a x4 write in 1x mode, the data2xn1x signal is driven low, andthe multiplexer 1302 which is controlled thereby outputs the data whichis received at its “0” input. When the write_m_active signal isasserted, the generation of act_stb_up[0:1] signals is enabled, but onlyif the second input to AND gate 1304 is asserted. During a 1x write ofany kind, the reset_L signal is held high. Thus, the second input to ANDgate 1304 will only be asserted when the output of mutiplexer 1306 ishigh. The output of multiplexer 1306 can only be asserted when thesignal write_x4 is asserted (since the multiplexer 1306 is controlled bythe output of multiplexer 1334). The write_x4 signal is therefore usedto indicate whether a write is being made to a DIMM comprised of x4 orx8/x16 DDR SDRAMs, and thus the write_x4 signal is responsive to valuesA_(x) output from the memory map 1100. If a write is being made to aDIMM comprised of x4 DDR SDRAMs, the write_x4 signal is asserted, and itis possible for the FIG. 13 circuitry to produce act_stb_up[0:1]signals. On the other hand, when a write is being made to DIMMscomprised of x8 or x16 DDR SDRAMs, the write_x4 signal is deasserted,and outputs act_stb_up[0:1] are held low.

The operation of the FIG. 13 circuitry during a x4 write in 1x modeproceeds as follows. With data2xn1x low, write_m_active high, andwrite_x4 high, the act_stb_up[0:1] signals remain low until the writesignal is asserted. After assertion of the write signal, the writesignal propagates through a path comprising multiplexers 1308 and 1306,OR gate 1310, D-type flip-flop 1312, AND gates 1304 and 1314,multiplexer 1302, D-type flip-flop 1316 and AND gate 1318 to therebyassert output act_stb_up[0]. The write signal also propagates throughmultiplexer 1320 and AND gate 1322 to thereby assert outputact_stb_up[1]. Thereafter, and so long as the inputs to circuit 1300 donot change state (but for clock MCK), the act_stb_up[0:1] signals willtoggle due to the presence of feedback path 1324.

A x4 write in 1x mode may be delayed by ½ MCK cycle by asserting the wpdsignal. Assertion of the wpd signal causes a ½ cycle strobe delay bycausing input 1326 to multiplexer 1320 to be delayed through D-typeflip-flop 1328.

A x4 write in 1x mode may also be subject to a long write preamble. Awrite preamble is a period of time prior to the generation of a strobesignal during which a strobe pad is held in a low state. Such a preamblemay be lengthened by asserting the long wpre signal, thereby causing thewrite and write_x4 signals to be respectively delayed through a pair ofD-type flip-flops 1330, 1332. Unlike assertion of the wpd signal, Whichonly delays a first rising edge of the act_stb_up[1] output, assertionof the long_wpre signal delays the first rising edges of both of theact_stb_up[0:1] signals.

As mentioned earlier in this description, a x8/x16 write in 1x moderesults in the act_stb_up[0:1] signals being held low, since upperstrobes are not necessary for a x8/x16 write. The upper strobes are heldlow by holding the write_x4 signal low.

The FIG. 13 circuitry operates in 2x mode as follows. The data2xn1xsignal is driven high, and gates 1318 and 1322 are disabled. As aresult, act_stb_up[0] and act_stb_up[1] are always held low in 2x mode.The reason that the act_stb_up[0:1] signals are not generated in 2x modeis that a choice was made to make intermediary chips 302 (FIG. 3)simpler by always writing to them as if they are x8 DIMMs. Thus fewersignals are routed to the intermediary chips 302, and when necessary,the intermediary chips 302 generate the additional strobes which theyneed to write to x4 DIMMs.

4. Write Timings

FIGS. 14–17 illustrate write timings of the memory controller drivercircuitry illustrated in FIGS. 7, 8, 12 and 13.

In FIG. 14, DQ4_PAD_ON asserts on the same cycle that DQ4 is driven. Asshown in FIG. 8, DQ4_PAD_ON is the signal which enables the tri-statebuffer coupled to the DQ4 pad. Likewise, DQS18_PAD_ON is the signalwhich enables the tri-state buffer coupled to the DQS18 pad. W1, Wbl-1and Wbl represent consecutive bits of a data word appearing at the DQ4pad. FIG. 14 illustrates the case where long_wpre=0, and hence specifiesa write preamble 1400 of one MCK clock cycle. FIG. 14 also assumes thatwpd=0. If wpd were asserted, then the DQS18, DQS18_PAD_ON, DQ4, andDQ4_PAD_ON signals would all shift to the right ½ MCK cycle (i.e., onephase). The signals connected to trk_pad_owd by arrows are allcontrolled by the assertion of trk_pad_owd, and therefore have a fixedtiming with respect to each other. The trk_pad_owd signal is a signalwhich causes a memory controller pad to “output write data”. The signalis generated in the core of memory controller 100 and is provided to apad control state machine 2900 (FIG. 29) for the purpose of generatingsignals trist_d and trist_s (FIG. 8). FIG. 14 applies to the writetimings of a lower strobe pad (i.e., a strobe pad that is configured towrite to DIMMs comprised of x4 and x8/x16 DDR SDRAMs). An upper strobepad (i.e., a strobe pad that is only configured to write to DIMMscomprised of x4 DDR SDRAMs) would hold the DQS18 line low when writesoccur to DIMMs comprised of x8/x16 DDR SDRAMs.

FIG. 15 is similar to FIG. 14, but with a long write preamble (i.e.,long_wpre=1). Hence, a write preamble 1500 of two MCK clock cycles isindicated.

In FIG. 16, DQ4_PAD_ON asserts on the same cycle that DQ4 is driven. w1,w2, . . . w8 represent consecutive bits of a data word appearing ath DQ4pad. FIG. 16 illustrates the case where long_wpre=0, and hence specifiesa write preamble 1600 of 0.75 MCK clock cycles. A 0.75 mck cyclepostamble is also provided. FIG. 16 also assumes that wpd=0. If wpd wereasserted, then the DQS18, DQS18_PAD_ON, DQ4, and DQ4_PAD_ON signalswould all shift to the right ½ MCK cycle (i.e., one phase). The signalsconnected to trk_pad_owd by arrows are all controlled by the assertionof trk_pad_owd, and therefore have a fixed timing with respect to eachother. FIG. 16 applies to write timings of a lower strobe pad. An upperstrobe pad would hold the DQS18 line low when writes occur to DIMMscomprised of x8 or x16 DDR SDRAMs.

FIG. 17 is similar to FIG. 16, but with a long write preamble (i.e.,long_wpre=1). Hence, a write preamble 1700 of 1.75 MCK clock cycles isindicated.

5. Ability to Read in 1x or 2x Mode

FIG. 18 illustrates a preferred embodiment of receiver circuitry 1800for a double data rate memory controller 100 (FIG. 9). The receivercircuitry 1800 comprises a data pad (DQ4), four transparent data inputlatches 1802, 1804, 1806, 1808, strobe distribution circuitry 1810, 1812(for distributing strobes to the data input latches 1802–1808 by meansof a count of strobe edges, thereby providing a means for reading datafrom a DIMM comprised of x4, x8 or x16 DDR SDRAMs), a pair of 4:1 deskewmultiplexers 1814, 1816 (i.e., a deskew multiplexing stage), andmultiplexer select logic 1818–1846 for providing the deskew multiplexers1814, 1816 with an appropriate set of control signals (depending onwhether the receiver circuitry 1800 is configured for a read in 1x modeor Mx (e.g., 2×) mode). Note that the data pad disclosed in FIG. 18 ispreferably synonymous with the data pad disclosed in FIG. 8, and is thusa bi-directional data pad.

The four data input latches 1802–1808 each receive the entire stream ofdata appearing at the data pad DQ4. However, the data input latches1802–1808 are made transparent one at a time, sequentially, and inresponse to a strobe signal received at a strobe pad DQS18 (FIG. 19) sothat 1) a first data bit is latched into latch 1802 in response to afirst incoming strobe edge received at the DQS18 strobe pad, 2) a seconddata bit is latched into latch 1804 in response to a second incomingstrobe edge received at the DQS18 pad, 3) a third data bit is latchedinto latch 1806 in response to a third incoming strobe edge received atthe DQS18 pad, 4) a fourth data bit is latched into latch 1808 inresponse to a fourth incoming strobe edge received at the DQS18 pad, andthen 5) new data bits are sequentially latched into latches 1802–1808again, beginning with latch 1802, if additional data bits need to bereceived at the DQ4 pad.

The generation of signals S1–S4 (or as can be seen in FIG. 18, signalsS1_alt–S4_alt) will be described in a later section of this description.For purposes of this section of the description, one need onlyunderstand that a set of control pulses such as signals S1–S4 areprovided for latching data into the respective data input latches1802–1808. Regardless of whether the receiver circuitry 1800 isconfigured to operate in 1x or 2x mode, and regardless of whether datais received from a DIMM comprised of x4 DDR SDRAMs, x8/x16 DDR SDRAMs,or an intermediary chip 302, control signals S1–S4 are sequentiallyasserted in response to incoming strobe edges received at a strobe pad.The S1–S4 clock pulses are therefore produced at a 1x or 2x DDR clockrate, depending on the rate at which strobe edges are received at acorresponding strobe pad.

As illustrated in FIG. 18, each of the transparent data input latches1802–1808 is coupled to inputs of first and second 4:1 deskewmultiplexers 1814, 1816. In 1x mode, however, the output of the seconddeskew multiplexer 1816 is meaningless and is ignored. As can be seen inthe figure, the deassertion of the data2xn1x signal holds the controlinputs of the second deskew multiplexer 1816 constant during a 1x moderead.

In a 1x mode burst of four read, four data bits are respectively clockedinto data input latches 1802–1808. Due to multiplexer select logiccomprising four D-type flip-flops, 1824, 1830, 1844, 1846, three gates1818, 1820, 1826, and two multiplexers 1822, 1828, the data inputs ofthe first deskew multiplexer 1814 are sequentially coupled to themultiplexer's output in the order 0, 2, 1, 3. The first deskewmultiplexer 1814 therefore outputs data bits sequentially, in the orderthey are received at the DQ4 data pad. Each output of the first deskewmultiplexer 1814 is clocked into the core clock domain of the FIG. 9memory controller on a rising edge of the controller's IOBCK clock(i.e., clocked through D-type flip-flop 1848 at a 1x DDR rate).

The control signals generated by the multiplexer select logic 1824,1830, 1844, 1846, 1818, 1820, 1826, 1822, 1828 in 1x mode are sometimesreferred to in the claims as a first set of control signals.

Note that the data input latches 1802–1808 operate in the time domain ofa strobe signal which is received at a strobe pad corresponding to thedata pad DQ4 (e.g., the strobe pad DQS18). However, the deskewmultiplexers 1814, 1816 operate in the clock domain of the memorycontroller 100. Due to the latching of data in four data input latches1802–1808, the memory controller 100 (and especially the multiplexerselect logic 1818–1846 for controlling the deskew multiplexers 1814,1816) is provided with a window equal to 1.5 periods of an incomingstrobe signal to clock data out of a data input latch and into the coreof the memory controller 100. One of ordinary skill in the art willreadily comprehend that the number of data input latches 1802–1808provided in the FIG. 18 receiver circuitry may be extrapolated to Platches, with P≧2, to thereby provide a shorter or longer period fortransferring data from the strobe domain of an incoming strobe signal tothe clock domain of the memory controller 100. One will also understandthat other kinds of storage elements may be used in lieu of transparentdata input latches 1802–1808 (e.g., D-type flip-flops).

A 1x mode burst of eight read operates similarly to a 1x mode burst offour read, with two sets of four data bits being latched into data inputlatches 1802–1808. The data inputs of the first deskew multiplexer 1814are therefore coupled to its output in the order 0, 2 1, 3, 0, 2, 1, 3.

In 2x mode, all reads are preferably executed as bursts of eight. Thedata input latches 1802–1808 are therefore made transparent similarly toa 1x mode burst of eight read, but at twice the rate. In 2x mode,however, both deskew multiplexers 1814, 1816 are active, with theirinputs being active in the following order:

input 0, multiplexer 1814

input 0, multiplexer 1816

input 1, multiplexer 1814

input 2, multiplexer 1816

Note that the control signals for both multiplexers 1814, 1816 changestate in sync with memory controller clock IOBCK, but that the controlsignals of multiplexer 1816 change state ½ IOBCK clock cycle out ofphase with the control signals for multiplexer 1814.

In 2x mode, the multiplexer select logic which controls the two deskewmultiplexers 1814, 1816 comprises five D-type flip-flops 1824, 1830,1836, 1842, 1844, four gates 1832, 1834, 1838, 1840, and twomultiplexers 1822, 1828. The control signals generated by themultiplexer select logic 1824, 1830, 1836, 1842, 1844, 1832, 1834, 1838,1840, 1822, 1828 in 2x mode are sometimes referred to in the claims as asecond set of control signals. Note that regardless of whether data bitsare received by the FIG. 18 circuitry 1800 in 1x or 2x mode, each of thecontrol signals which are generated by the multiplexer select logic1818–1846 may be generated at a 1x rate, even though together, thedeskew multiplexers 1814, 1816 effectively produce data at a 2x rate.

Due to the two alternately clocked D-type flip-flops 1850, 1852 whichare coupled to the output of the second deskew multiplexer 1816, evenand odd 2x data bits are output to the core of the memory controller 100in parallel. This fact is merely a design choice, and is only providedfor completeness of the preferred embodiment's description.

The FIG. 18 receiver circuitry may be enabled and disabled via AND gate1854. The AND gate 1854 is enabled and disabled via the output of amultiplexer 1856, which outputs the signal DQ4_RCV_ON after ½ or 1 cycleof clock IOBCK (as determined by a pair of cascaded D-type flip-flops1858, 1860 and the state of the read phase delay (rpd) signal). Thepurpose of the AND gate 1854 and its associated logic 1856–1860 is toshield downstream receiver circuitry 1802–1808, 1814, 1816, 1848–1852from noise that could be present when the bus coupled to data pad DQ4 isidle and tri-stated.

6. Tri-state Noise Immunity on Reads

During a memory read cycle, there is a need to account for variation incontroller-memory-controller loop delay (i.e., read loop delay). Forexample, in FIG. 21 a plurality of memory modules 104 is coupled to amemory controller 100 over common data (DQ) and strobe (DQS) buses. Notonly is a plurality of memory modules 104 coupled to the data and strobebuses, but each of the memory modules 104 may exhibit timing variationswithin allowed ranges (e.g., within the ranges provided in the JEDEC DDRSDRAM Specification). Furthermore, copies of a clock signal which aredistributed to each of the plurality of memory modules 104 may becomeskewed with respect to one another.

As a result of the above irregularities, read requests which aredispatched to different memory modules (with their varied timingcharacteristics and skewed clocks) can take varying amounts of time toreturn to the controller, and there is a variation in read loop delaywhich needs to be accounted for when determining when to enable anddisable the receipt of data and strobe signals at a memory controller100. Such a delay can only be accounted for by ensuring that a memorycontroller 100 will appropriately receive data and strobes in responseto a shortest possible loop delay (i.e., an early receipt case) and alongest possible loop delay (i.e., a late receipt case).

The data and strobe buses for memory modules 104 under the JEDEC DDRSDRAM Specification have a notable characteristic. The reference voltagefor each bus line is the same as the bus line's termination voltage.What this means is that, as a result of noise, the strobe pads of amemory controller 100 are subject to erroneous “0” to “1” and “1” to “0”transitions when their corresponding bus lines are tri-stated. If notaccounted for, these transitions can be erroneously interpreted asactive strobe edges, thereby leading to potential data corruption.

The JEDEC DDR SDRAM specification attempts to overcome this problem byproviding a “read preamble” and “read postamble”. The read preambleprovides a period of time before the first incoming strobe edge duringwhich a strobe pad is held low. Likewise, the read postamble provides aperiod of time after the last incoming strobe edge during which a strobepad is held low. As a result, strobe edges can arrive at a strobe padsomewhat early or somewhat late and still fall within the period which amemory controller 100 allots for the return of a read cycle. However,problems can still arise when the variation between early and latearriving strobe edges is great.

FIG. 22 illustrates a DDR read cycle in 1x mode (data2xn1×=0) with noread phase delay (rpd=0), as seen by the FIG. 18 and 20 receivercircuitry 1800, 2000. Note the variation between signals DQS18 (early)and DQS18 (late), which respectively represent the earliest and latestexpected arrival of strobe edges at strobe pad DQS18 (i.e., an “earlyreceipt case” and a “late receipt case”). Note also that the “0” totri-state strobe transition in the early receipt case occurs before thelast strobe edge is received in the late receipt case. Therefore, unlessone can predict in advance exactly when strobe signals will be receivedat a strobe pad (i.e., early or late), and make such a prediction withperfect accuracy, then one cannot, in all cases, tri-state a strobepad's receiver both 1) after its receipt of a last incoming strobe edge,and 2) before an incoming strobe signal tri-states. As one of ordinaryskill in the art will readily comprehend, predicting when strobe signalswill be received at a strobe pad is extremely difficult, as such aprediction depends not only on wire routes between a controller 100 andmemory 104, as well as the time it takes to access a particular memoryaddress within a memory module, but also on temperature, clock skew,memory access speeds, and so on. A way of preventing the tri-stating ofa strobe signal from producing noise which is interpreted as activestrobe edges is therefore needed. FIGS. 18, 19 & 20 illustrate such ameans.

In FIG. 18, four data input latches 1802–1808 are coupled to receivedata from data pad DQ4. The four latches 1802–1808 are respectivelycontrolled by values of a count. For example, in FIG. 18, the fourlatches 1802–1808 are controlled by a four bit, one-high countcomprising bits S1, S2, S3 and S4. This count may be produced by thecounter 1900 illustrated in FIG. 19, in response to buffered strobeedges output from a receiver 2030 which is coupled to the strobe padDQS18. The counter is controlled by counter control logic 1902comprising a block of combinational logic 1904 and a portion of thememory controller's core logic 1908. The combinational logic 1904receives a control signal (DQS18_start), feedback 1906 from the counter1900, and the enable signal DQS18_RCV_ON. The enable signal DQS18_RCV_ONis provided to both the combinational logic 1904 of the counter controllogic 1902, as well as the receiver 2030 which buffers strobe edgesreceived at strobe pad DQS18, so that the strobe receiver circuitryshown in FIG. 19 may be globally enabled similarly to the way in whichthe FIG. 18 data receiver circuitry is enabled.

The combinational logic's primary control inputs are the control signalDQS18_start and the counter feedback 1906. Assuming that 1) DQS18_RCV_ONis asserted, and 2) the counter 1900 is in reset, then the controlsignal DQS18_start determines when the counter 1900 is enabled. In afirst preferred embodiment, the control signal DQS18_start is merely apulse of fixed width which is generated prior to each read cycle of thememory controller 100. In this first preferred embodiment, each strobesignal received at DQS18 is presumed to have the same number of edges.In a second preferred embodiment, the control signal DQS18_startcomprises a start condition (e.g., a falling edge) and a stop condition(e.g., a rising edge), with the timing of the start and stop conditionsvarying depending on the number of strobe edges which are expected to bereceived during a current read cycle. In this manner, the core logic1908 can time the start and stop conditions depending on whether acurrent read cycle is, for example, 1) a DDR burst of four or burst ofeight read cycle, or 2) a 2x mode or Mx mode DDR read cycle (M≧2). Thelatter embodiment of the control signal DQS18_start therefore provides amemory controller 100 with greater read flexibility.

FIG. 20 illustrates a preferred and more detailed embodiment of thecircuitry illustrated in FIG. 19, wherein signals S1–S4 are produced bya rollover counter 2002–2012 which increments its four bit, one-highcount in response to each strobe edge received at strobe pad DQS18. Therollover counter 2002–2012 is enabled and reset by counter control logic2014–2028 which is coupled to DQS18_tff_rise_rst and DQS18_tff_fall_rstinputs of the counter 2002–2012. During a read cycle, the countercontrol logic 2014–2028 generates a start condition at DQS18_tff_start(a falling edge in FIG. 20), to thereby enable the counter's counting ofstrobe edges. That is, assuming that the counter 2002–2012 is already inreset. The counter control logic 2014–2028 then assists in resetting thecounter 2002–2012 by generating a stop condition (a rising edge in FIG.20) at DQS18_tff_start.

As will be explained in more detail below, the start condition serves toenable the counter 2002–2012 asynchronously with respect to the strobeedges which are received at strobe pad DQS18.

The counter 2002–2012 is also asynchronously reset with respect tostrobe edges received at strobe pad DQS18. The counter 2002–2012 isreset in response to the stop condition and counter feedback. Note thatin FIG. 22, the stop condition is generated in the midst of a readcycle, and during the counter's counting of a read cycle's last fourstrobe edges (i.e., a last P strobe edges in the claims). However, dueto counter feedback received at inputs of logic gates 2014 and 2018 ofthe counter control logic 2014–2028, the counter 2002–2012 continuescounting the last four strobe edges of a received strobe signal beforeentering a reset state—even though the counter 2002–2012 has alreadyreceived a stop condition. The stop condition therefore does notimmediately stop the counter 2002–2012, but rather prevents the counter2002–2012 from counting past the last four strobe edges of a receivedstrobe signal. As will be understood shortly, the counter 2002–2012 willcount the last four strobe edges regardless of where it is in its countwhen a stop condition is generated. Thus, regardless of whether thecounter 2002–2012 has counted one, two or three of the last four strobeedges when a stop condition is generated, the counter will finishcounting the last four strobe edges of a strobe signal and then stopcounting. As a result, so long as the last four strobe edges of DQS18(early) overlap the last four strobe edges of DQS18 (late), a time canbe found to assert DQS18_tff_start such that 1) all strobe edges will becounted, and 2) the counter 2002–2012 will be reset prior to when astrobe bus is tri-stated.

In FIG. 20, the rollover counter comprises two state elements 2002,2004. The state elements are preferably toggle flip-flops 2002, 2004which produce an arithmetic binary count (SA:SB). The counter alsocomprises combinational logic 2006–2012 which converts theafore-mentioned arithmetic binary count to a four bit, one-high binarycount.

The first of the two flip-flops 2002 produces outputs SA and SA′ and isclocked by rising strobe edges. The second of the two flip-flops 2004produces outputs SB and SB′ and is clocked by falling strobe edges.

Downstream from the counter's two flip-flops 2002, 2004, thecombinational logic which converts the flip-flops' arithmetic binarycount to a one-high binary count comprises four AND gates 2006–2012. Theinputs of the four AND gates 2006–2012 are tied to various ones of theoutputs SA, SA′, SB and SB′ such that the AND gates 2006–2012 asserttheir outputs S1–S4 in a sequential and rollover manner.

If the flip-flop 2002 which produces output SA is considered to be thelow order flip-flop of the counter 2002–2012, then the binary countwhich is produced by the counter's flip-flops 2002, 2004 will assume thefollowing sequence: 0, 1, 3, 2, 0, 1, 3, 2, 0, . . . . Thus, the orderof the counter's count is not as important as the consistent andrepetitive nature of the counter's count. Also, although the countershown in FIG. 20 is a rollover counter 2002–2012, the counter can takeother forms. For example, the counter could comprise additional stateelements 2002, 2004 or output logic 2006–2012 for counting all strobeedges of a strobe-signal, without needing to roll over during a givencount.

The counter control logic 2014–2028 which enables and resets the countercomprises a pair of AND gates 2014, 2018 which are respectively coupledto the reset inputs of the two flip-flops 2002, 2004 via an optionalpair of OR gates 2016, 2020. The purpose of the optional OR gates 2016,2020 will be described shortly. By means of a first AND and OR gate2014, 2016, the first flip-flop's reset input is defined by theequation:SA′·DQS18_tff_start (where “·” indicates a logical AND operation).

By means of a second AND and OR gate 2018, 2020, the second flip-flop'sreset input is determined by the equation:SA′·SB′.

Thus, each of the AND gates 2014, 2018 receives feedback from thecounter 2002–2012 (i.e., “counter feedback”). The first AND gate 2014,however, also receives the start and stop conditions which are generatedat DQS18_tff_start.

The start and stop conditions which assist in respectively enabling andresetting the counter 2002–2012 are generated on the single signal linelabeled DQS18_tff_start. A start condition is denoted by a falling edgeat DQS18_tff_start, and a stop condition is denoted by a rising edge atDQS18_tff_start. The start and stop conditions are generated by logiccomprising a multiplexer 2028, an AND gate 2026, and two alternatelyclocked D-type flip-flops 2022, 2024. The flip-flops 2022, 2024 and ANDgate 2026 are coupled such that the multiplexer 2028 receives the signalstb_reset at each of its inputs, but receives changes in the stb_resetsignal at its “0” input ½ IOBCK cycle after it receives changes in thestb_reset signal at its “1” input. The state of the multiplexer 2028 iscontrolled by the read phase delay signal (rpd), and as a result, achange in stb_reset is reflected at DQS18_tff_start 1/2 IOBCK cyclelater when there is a read phase delay (i.e., when rpd=1).

During a read cycle, each of the counter control logic's two OR gates2008, 2012 receives a logic “0” derived from the inverse of theDQS18_RCV_ON signal. One will note that DQS18_RCV_ON is the signal whichcontrols the receiver 2030 coupled to the DQS18 strobe pad. DQS18_RCV_ONis therefore asserted during a read cycle, and deasserted between readcycles. By coupling DQS18_RCV_ON to the counter's two OR gates 2008,2012 via an inverter 2032, an extra safety measure is provided to ensurethat none of the counter's outputs (i.e., S1–S4) is assertedunintentionally. Furthermore, the assertion of DQS18_RCV_ON can be usedto reset the counter 2002–2012 upon power on.

The state of signal DQS18_RCV_ON is controlled similarly to the state ofsignal DQS18_tff_start. That is, the state of DQS18 RCV_ON is controlledby two alternately clocked D-type flip-flops 2034, 2036 which arecoupled to the inputs of a multiplexer 2038, which multiplexer iscontrolled by the rpd signal. Thus, a change in the state ofDQS18_RCV_ON will be delayed by ½ IOBCK cycle when there is a read phasedelay.

One skilled in the art will readily comprehend that the FIGS. 18 & 20circuitry can be extrapolated to receive data bits and strobe edgesconsisting of a multiple of P data bits and strobe edges. In such anextrapolation, the counter 2002–2012 is expanded to produce a P bit,one-high count. Likewise, the number of data input latches 1802–1808 maybe expanded to P latches. Thus, in FIGS. 18 & 20, P=4.

The operation of the receiver circuitry 2000 illustrated in FIG. 20 canbe better understood by referring to FIGS. 22–27. FIG. 22 provides acomparison of various signal timings for early and late case reads in 1xmode with no read phase delay (rpd=0). Similarly, FIG. 25 provides acomparison of various signal timings for early and late case reads in 2xmode with no read phase delay (rpd=0). FIGS. 23, 24, 26 and 27 show thefollowing:

FIG. 23: signal timings in 1x mode, early case, rpd=0

FIG. 24: signal timings in 1x mode, late case, rpd=0

FIG. 26: signal timings in 2x mode, early case, rpd=0

FIG. 27: signal timings in 2x mode, late case, rpd=0

A. 1x Read Cycles

Referring now to FIG. 22, there is shown the core clock, MCK, of thememory controller 100 illustrated in FIGS. 9, 18 and 20. The signalstrk_ird and trk_srd are provided to a pad control state machine 2900(FIG. 29) which respectively generates the stb_reset signal provided toflip-flop 2022, as well as the rcv_on signal provided to flip-flop 2034.The trk_srd and trk_ird signals therefore determine the rise and fall ofvarious strobe receiver circuitry signals. Although not shown in FIG.29, the pad control state machine also receives signals which indicatewhether a current read cycle is to assume a 1x or 2x mode, and a burstof 4 or burst of 8 mode. These additional signals, in turn, determinewhen stb_reset and rcv_on are asserted.

As illustrated in FIG. 22, the assertion of the trk_ird signaldetermines the rise of DQS18_RCV_ON and the fall of DQS18_tff_start(with the fall of DQS18_tff_start representing the afore-mentioned startcondition). If the counter 2002–2012 is in reset (as it should be whentrk_ird is asserted), then the assertion of trk_ird also determines thefall of DQS18_tff_rise_rst.

Note that the fall of DQS18_RCV_ON enables the FIG. 20 strobe receivercircuitry 2000 in general, but does not enable the circuitry's counter2002–2012. However, with the fall of DQS18_tff_start andDQS18_tff_rise_rst one cycle after the rise of DQS18_RCV_ON, the counter2002–2012 is placed in a state wherein it is ready to begin countingstrobe edges as soon as they are received (i.e., the counter 2002–2012is asynchronously enabled in response to a start condition and counterfeedback).

Preferably, the counter 2002–2012 is enabled at a time falling betweeni) a latest time when the counter control logic 2014–2028 expects astrobe bus coupled to strobe pad DQS18 to leave tri-state, and ii) anearliest time when the counter control logic 2014–2028 expects edges ofa strobe signal to be received at strobe pad DQS18. Thus, with respectto the strobe receiver circuitry's receipt of strobes conforming to theJEDEC DDR SDRAM Specification, the counter 2002–2012 is preferablyenabled at a time in which the counter control logic 2014–2028 expectsboth DQS18 (early) and DQS18 (late) to be in their preamble state. Inthis manner, erroneous transitions at strobe pad DQS18 as a result oftri-state noise will not be interpreted by the counter 2002–2012 asactive strobe edges.

The assertion of the trk_srd signal determines the rise ofDQS18_tff_start, as well as the fall of DQS18_RCV_ON (which falldisables the FIG. 20 strobe receiver circuitry 2000 in its entirety).The rise of DQS18_tff_start and fall of DQS18_RCV_ON vary in timingdepending on whether a read cycle is a burst of four or burst of eightcycle, as controlled by the pad control state machine 2900. As will benoted during this description's discussion of a 2x mode read cycle, thetiming of the trk_srd control pulse is fixed in 2x mode as a result of adecision being made to only support burst of eight reads in 2x mode.

Note that during a 1x mode burst of four read cycle, DQS18_tff_start maybe timed to rise anytime between the first rising edge of DQS18 (late)and the end of the DQS18 (early) postamble. So long as DQS18_tff_startrises during this period, the counter 2002–2012 will count each andevery active strobe signal received at DQS18 and then asynchronouslyreset itself in response to the rise of DQS18_tff_start (i.e., a stopcondition) and counter feedback. Phantom strobe edges which are producedas a result of noise as DQS18 (early) tri-states will therefore not becounted by the counter 2002–2012. Consequently, the four data inputlatches 1802–1808 which are coupled to data pad DQ4 will not be clockedinadvertently.

The fall of DQS18_tff_start, the fall of DQS18_tff_rise_rst, and therise of DQS18_RCV_ON are all triggered in the clock domain of memorycontroller 100. All other rising and falling edges within the strobereceiver circuitry 2000 are triggered off of strobe edges received atthe DQS18 strobe pad, and are therefore triggered in the strobe domainof the memory modules 104 or intermediate chips 302 to which the DQS18strobe pad is attached.

As shown in FIG. 22, the first falling edge of DQS18_tff_fall_rst istriggered off of the FIG. 20 counter's receipt of a first strobe edge.Thereafter, the counter 2002–2012 continues to count strobe edges untilsuch time that the DQS18_tff_start signal is asserted. After this point,and upon reaching count “3” (i.e., the third of the last four strobeedges), the rise of DQS18_tff_rise_rst is asynchronously triggered inresponse to counter feedback. Likewise, and upon reaching count “2”, therise of DQS18_tff_fall_rst is asynchronously triggered as a result ofcounter feedback. Once DQS18_tff_rise_rst and DQS18_tff_fall_rst rise,they are prevented from falling so long as DQS18_tff_start is held high(i.e., until a next read cycle is initiated).

For completeness, FIG. 22 illustrates the receipt of data at data padDQ4 (FIG. 18) in an “early receipt case” and “late receipt case” (i.e.,DQ4 (early) and DQ4 (late)). FIG. 22 also illustrates the period overwhich the DQ4 data pad is enabled for receiving data (denoted by theassertion of signal DQ4_RCV_ON).

FIG. 23 illustrates signal timings in the 1x mode early read case withno read phase delay (rpd=0). Note that the overlapping assertions anddeassertions of flip-flop outputs SA and SB lead to a production ofS1–S4 signals with consecutive pulses. Each S1–S4 pulse is produced atthe frequency of an incoming strobe signal received at strobe pad DQS18.

FIG. 24 illustrates signal timings in the 1x mode late read case with noread phase delay (rpd=0).

B. 2x Read Cycles

Referring now to FIG. 25, there is once again shown the core clock, MCK,of the memory controller 100 illustrated in FIGS. 9, 18 and 20.

As illustrated in FIG. 25, the assertion of the trk_ird signaldetermines the rise of DQS18_RCV_ON and the fall of DQS18_tff_start(with the fall of DQS18_tff_start representing the afore-mentioned startcondition). If the counter is in reset (as it should be when trk_ird isasserted), then the assertion of trk_ird also determines the fall ofDQS18_tff_rise_rst.

Note that the fall of DQS18_RCV_ON enables the FIG. 20 strobe receivercircuitry 2000 in general, but does not enable the circuitry's counter2002–2012. However, with the fall of DQS18_tff_start andDQS18_tff_rise_rst one cycle after the rise of DQS18_RCV_ON, the counter2002–2012 is placed in a state wherein it is ready to begin countingstrobe edges as soon as they are received (i.e., the counter 2002–2012is asynchronously enabled in response to a start condition and counterfeedback).

Preferably, the counter 2002–2012 is enabled at a time falling betweeni) a latest time when the counter control logic 2014–2028 expects astrobe bus coupled to strobe pad DQS18 to leave tri-state, and ii) anearliest time when the counter control logic 2014–2028 expects edges ofa strobe signal to be received at strobe pad DQS18. Thus, with respectto the strobe receiver circuitry's receipt of strobes conforming to theJEDEC DDR SDRAM Specification, the counter 2002–2012 is preferablyenabled at a time in which the counter control logic 2014–2028 expectsboth DQS18 (early) and DQS18 (late) to be in their preamble state. Inthis manner, erroneous transitions at strobe pad DQS18 as a result oftri-state noise will not be interpreted by the counter 2002–2012 asactive strobe edges.

The assertion of the trk_srd signal determines the rise ofDQS18_tff_start, as well as the fall of DQS18_RCV_ON (which falldisables the FIG. 20 strobe receiver circuitry 2000 in its entirety).Since the memory controller 100 does not communicate directly withmemory 104 in 2x mode, 2x reads preferably always assume a burst ofeight form, and thus the rise of DQS18_tff_start and fall ofDQS18_RCV_ON have fixed timings in 2x mode.

Note that during a 2x mode read cycle, DQS18_tff_start may be timed torise anytime between the third rising edge of DQS18 (late) and the endof the DQS18 (early) postamble. So long as DQS18_tff_start rises duringthis period, the counter 2002–2012 will count each and every activestrobe signal received at DQS18 and then asynchronously reset itself inresponse to the rise of DQS18_tff_start (i.e., a stop condition) andcounter feedback. Phantom strobe edges which are produced as a result ofnoise as DQS18 (early) tri-states will therefore not be counted by thecounter 2002–2012. Consequently, the four data input latches 1802–1808which are coupled to data pad DQ4 will not be clocked inadvertently.

As in 1x mode, the rise of DQS18_RCV_ON, the fall of DQS18_tff_start,and the fall of DQS18_tff_rise_rst are all triggered in the clock domainof memory controller 100. All other rising and falling edges within thestrobe receiver circuitry 2000 are triggered off of strobe edgesreceived at the DQS18 strobe pad, and are therefore triggered in thestrobe domain of the memory modules 104 or intermediate chips 302 towhich the DQS18 strobe pad is attached.

As shown in FIG. 25, the first falling edge of DQS18_tff_fall_rst istriggered off of the FIG. 20 counter's receipt of a first strobe edge.Thereafter, the counter 2002–2012 continues to count strobe edges untilsuch time that the DQS18_tff_start signal is asserted. After this point,and upon reaching count “3” (i.e., the third of the last four strobeedges), the rise of DQS18_tff_rise_rst is asynchronously triggered inresponse to counter feedback. Likewise, and upon reaching count “2”, therise of DQS18_tff_fall_rst is asynchronously triggered as a result ofcounter feedback. Once DQS18_tff_rise_rst and DQS18_tff_fall_rst rise,they are prevented from falling so long as DQS18_tff_start is held high(i.e., until a next read cycle is initiated).

For completeness, FIG. 25 illustrates the receipt of data at data padDQ4 (FIG. 18) in an “early receipt case” and “late receipt case” (i.e.,DQ4 (early) and DQ4 (late)). FIG. 22 also illustrates the period overwhich the DQ4 data pad is enabled for receiving data (denoted by theassertion of signal DQ4_RCV_ON).

FIG. 26 illustrates signal timings in the 2x mode early read case withno read phase delay (rpd=0). Note that the overlapping assertions anddeassertions of flip-flop outputs SA and SB lead to a production ofS1–S4 signals with repetitive and consecutive pulses. Each pulse isproduced at the frequency of a strobe signal received at the DQS18strobe pad, ane each pulse latches a new data bit (i.e., r1, r2, r3, r4,r5, r6, r7 or r8) into the DQ4 receiver circuitry 1800 (see FIG. 18).

FIG. 27 illustrates signal timings in the 2x mode late read case with noread phase delay (rpd=0).

7. Ability to Read DIMMs Comprised of x4, x8 and x16 RAM Devices

By means of the memory map 1100 illustrated in FIG. 11, the data/strobepairings illustrated in FIGS. 9 & 10, and the control circuitry 1810,1812, 2800–2806 illustrated in FIGS. 18 & 28 (which control circuitryhas yet to be discussed), the memory controller 100 illustrated in FIG.9 is able to read data from memory modules 104 comprised ofnon-homogeneous data width RAM devices (e.g., DIMMs comprised of x4 DDRSDRAMs, DIMMs comprised of x8 DDR SDRAMs, and DIMMs comprised of x16 DDRSDRAMs).

As previously discussed with respect to writes of DIMMs 104 comprised ofnon-homogeneous data width RAM devices, the memory map 1100 stores anindication of a data/strobe ratio (e.g., a value A_(x)) for each memorymodule x which is coupled to the memory controller 100. During a readcycle of the memory controller 100, an addressed value A_(x) is outputfrom the memory map 1100 and provided to control circuitry 1810, 1812,2800–2806 which controls data receipt at a subset of the memorycontroller's data pads. In FIG. 18, the control circuitry is shown tocomprise a number of multiplexers 1812, each of which receives anaddressed value A_(x) (in the form of signal set_alt_n) as a controlinput.

In general, the control circuitry 1810, 1812, 2800–2806 controls datareceipt at a subset of the memory controller's data pads as is discussedin previous sections of this description. That is, a count of strobeedges received at one of a memory controller's strobe pads (e.g., DQS18)is used to control the data storage elements (e.g., data latches1802–1808) coupled to one or more of the memory controller's data pads(e.g., DQ4–DQ7; FIGS. 9 & 18). For example, four data input latches1802–1808 may be coupled to each data pad of a memory controller 100,and the bits of a four bit, one-high strobe edge count may be used tocontrol respective ones of the latches 1802–1808. Likewise, each bit ofa four-bit, one-high strobe edge count may control corresponding datainput latches 1802–1808 coupled to each of a number of data pads (e.g.,pads DQ4–DQ7).

While the above paragraph has summarized a preferred and previouslydescribed method of controlling the receipt of data at a number of datapads, the afore-mentioned method of controlling data receipt can beaccomplished without the multiplexer 1812 which couples the data andstrobe receiver circuits illustrated in FIGS. 18 & 20. The additionalfunctionality provided by the multiplexer 1812 coupling the FIG. 18 datareceiver circuit 1800 and FIG. 20 strobe receiver circuit 2000 is theability to associate a data receiver circuit 1800 with two or moredifferent strobe receiver circuits 2000. In this manner, the receipt ofdata at some of a memory controller's data pads may be controlled by oneof a plurality of different strobes, and functionality such as theability to read from DIMMs comprised of x4, x8 and x16 DDR SDRAMs isprovided.

In FIG. 18, the receipt of data at data pad DQ4 may be controlled inresponse to a strobe received at strobe pad DQS18, or a strobe receivedat an alternate strobe pad. Referring to the associations of data andstrobe pads provided in FIG. 10, one can appreciate that a read from aDIMM comprised of x4 DDR SDRAMs requires an association between data padDQ4 and strobe pad DQS18, while a read from a DIMM comprised of x8 orx16 DDR SDRAMs requires an association between data pad DQ4 and strobepad DQS18. Thus, the strobe edge count comprised of bits S1_alt–S4_altin FIG. 18 is a count of strobe edges produced at strobe pad DQS18.

When extrapolating the circuitry 1800, 2000 illustrated in FIGS. 18 & 20to a larger scale, one skilled in the art will realize that the FIG. 9memory controller 100 is provided with an ability to read DIMMs 104comprised of non-homogeneous DDR SDRAMs by 1) providing a fixedcorrelation between a first half of the memory controller's data pads(DQ0–DQ3, DQ8–DQ11, DQ16–19, . . . ) and the memory controller's lowerstrobe pads (DQS0–DQS17), and 2) providing a programmable correlationbetween a second half of the memory controller's data pads (DQ4–DQ7,DQ12–DQ15, DQ20–23, . . . ) and the memory controller's upper(DQS18–DQS35) and lower (DQS0–DQS17) strobe pads. In the latter case,the programmable correlation is determined by control circuitrycomprising, for example, a plurality of multiplexers 1812 which receiveaddressed values of A_(x) (appearing in FIG. 18 as signal set_alt_n)from the memory map 1100. If an addressed value A_(x) is a logic “1”,then the data and strobe pads will be associated in a manner whichallows for reading data from DIMMs comprised of x4 DDR SDRAMs (i.e., acount (bits S1–S4) which is received at the multiplexer's first datainput will be passed through the multiplexer 1812). If an addressedvalue A_(x) is a logic “0”, then the data and strobe pads will beassociated in a manner which allows for reading data from DIMMscomprised of x8 or x16 DDR SDRAMs (i.e., a count (bits S1_alt–S4_alt)which is received at the multiplexer's second data input will be passedthrough the multiplexer 1812).

In a preferred embodiment, the critical signal for controlling themultiplexer 1812 is the set_alt_n signal. A possible derivation of thissignal is illustrated in FIG. 28. After reset_L is briefly driven lowduring system reset, reset_L is held high. The selection of a primary oralternate strobe edge count for the purpose of controlling data inputlatches 1802–1808 is therefore determined by the signals read_tri andread_tri_x4. During a read of a DIMM comprised of x4 DDR SDRAMs, bothread_tri and read_tri_x4 are asserted, and multiplexer 2800, OR gate2802, and D-type flip-flops 2804 and 2806 assert the signal set_alt_n.However, during a read of a DIMM comprised of x8 or x16 DDR SDRAMs,read_tri_x4 is deasserted to thereby deassert the set_alt_n signal. Inthis manner the FIG. 9 memory controller may be configured to read datafrom DIMMs comprised of x4, x8 and x16 DDR SDRAMs. The read_tri_x4signal may be, for example, an addressed value A_(x) or a derivativethereof.

Although the memory controller 100 which is described above is capableof communicating with DIMMs 104 comprised of DDR SDRAMs having twodifferent data/strobe ratios (i.e., 4:1 and 8:1 ratios), the teachingsprovided herein may be adapted to provide even greater flexibility forreading from memory modules 104 comprised of non-homogeneous data widthRAM devices. For example, the control circuitry 1810, 1812, 2800–2806for associating data and strobe driver circuits 1800, 2000 may comprisemultiplexers which receive data based on strobes received at more thantwo strobe pads (e.g:, strobe edge counts based on strobes received atmore than two strobe pads). Furthermore, the subset of a memorycontroller's data pads which have a fixed correlation with ones of thememory controller's strobe pads may be greater, smaller, or evennon-existent.

While illustrative and presently preferred embodiments of the inventionhave been described in detail herein, it is to be understood that theinventive concepts may be otherwise variously embodied and employed, andthat the appended claims are intended to be construed to include suchvariations, except as limited by the prior art.

1. A memory controller, comprising: a plurality of data pads and strobe pads, wherein data is read and written at each of said data pads in sync with a strobe that is received at or generated by a corresponding one of said strobe pads; for each data pad, receiver circuitry comprising P storage elements, wherein i) in a first mode, data bits stored by the P storage elements are multiplexed to generate a single data stream, and ii) in a second mode, data bits stored in the P storage elements are multiplexed to generate at least two data streams; and for each strobe pad, receiver circuitry comprising a counter to count received strobe edges; wherein particular counts of strobe edges received at a particular strobe pad cause data to be received by particular ones of the P storage elements associated with data pads corresponding to the particular strobe pad.
 2. The memory controller of claim 1, wherein each of said storage elements is a latch.
 3. The memory controller of claim 1, wherein each of said storage elements is a flip-flop.
 4. The memory controller of claim 1, wherein each of said counters is a rollover counter which produces said count as a P bit, one-high count.
 5. The memory controller of claim 1, wherein P=4.
 6. The memory controller of claim 1, wherein the at least two data streams consist of even and odd data streams.
 7. The memory controller of claim 1, further comprising: a clock generation circuit to generate strobes at said strobe pads at a 1x rate when the memory controller is configured in said first mode, or at an Mx rate when the memory controller is configured in said second mode; for each data pad, driver circuitry to alternately couple ones of a corresponding subset of N data propagation circuits to the data pad, thereby driving a merged data stream to the data pad; and circuitry to, i) in said first mode, provide like data input streams to each of the N data propagation circuits associated with a given data pad, and ii) in said second mode, provide different data input streams to each of the N data propagation circuits associated with a given data pad.
 8. The memory controller of claim 7, wherein for each data pad, said driver circuitry comprises N sequentially clocked flip-flops which respectively receive and output data from the N data propagation circuits.
 9. The memory controller of claim 7, wherein for each data pad, said driver circuitry comprises a multiplexer which receives and sequentially outputs data from the N data propagation circuits.
 10. The memory controller of claim 7, wherein for each data pad, said driver circuitry comprises N tri-statable paths which respectively receive and sequentially output data from the N data propagation circuits.
 11. The memory controller of claim 1, further comprising a memory to store indications of data/strobe ratios that are required to access memory devices that are coupled to said data and strobe pads of the memory controller; wherein, for a data transmission initiated with a particular one of the memory devices, a number of said data pads is dynamically associated with a number of said strobe pads, in response to a corresponding indication of a data/strobe ratio stored in the memory.
 12. A computer system, comprising: a CPU; a memory controller coupled to said CPU; and a number of memory devices coupled to said memory controller; wherein said memory controller comprises: a plurality of data pads and strobe pads coupled to said memory devices, wherein data is read and written at each of said data pads in sync with a strobe that is received at or generated by a corresponding one of said strobe pads; for each data pad, receiver circuitry comprising P storage elements, wherein i) in a first mode, data bits stored by the P storage elements are multiplexed to generate a single data stream, and ii) in a second mode, data bits stored in the P storage elements are multiplexed to generate at least two data streams; and for each strobe pad, receiver circuitry comprising a counter to count received strobe edges; wherein particular counts of strobe edges received at a particular strobe pad cause data to be received by particular ones of the P storage elements associated with data pads corresponding to the particular strobe pad.
 13. The computer system of claim 12, wherein each of said counters is a rollover counter which produces said count as a P bit, one-high count.
 14. The computer system of claim 12, further comprising: a clock generation circuit to generate strobes at said strobe pads at a 1x rate when the memory controller is configured in said first mode, or at an Mx rate when the memory controller is configured in said second mode; for each data pad, driver circuitry to alternately couple ones of a corresponding subset of N data propagation circuits to the data pad, thereby driving a merged data stream to the data pad; and circuitry to, i) in said first mode, provide like data input streams to each of the N data propagation circuits associated with a given data pad, and ii) in said second mode, provide different data input streams to each of the N data propagation circuits associated with a given data pad.
 15. The computer system of claim 12, further comprising a memory to store indications of data/strobe ratios that are required to access memory devices that are coupled to said data and strobe pads of the memory controller; wherein, for a data transmission initiated with a particular one of the memory devices, a number of said data pads is dynamically associated with a number of said strobe pads, in response to a corresponding indication of a data/strobe ratio stored in the memory. 